The present invention relates to a hydraulic circuit including a relief valve device for reducing a main circuit pressure when the main circuit pressure exceeds a predetermined pressure in the hydraulic circuit including a hydraulic pump.
In the related art, in a hydraulic circuit including flow rate control means for controlling a discharge flow rate (discharge pressure) of a hydraulic pump in the hydraulic circuit including the hydraulic pump, when sharp fluctuations occur in the discharge flow rate of the hydraulic pump, there has been a possibility that a surge pressure is generated in a main circuit to cause damage to hydraulic equipment, pipes, fittings and hydraulic pump drive devices. Since the surge pressure generated at that time is momentarily high, even if a fixed relief valve in which a specified valve opening pressure has been set is provided in the hydraulic circuit, there has been a problem in that it is difficult to remove the surge pressure.
Specifically, referring to FIGS. 5 and 6, the following will be described. FIG. 5 illustrates one configuration example of the hydraulic circuit in the related art, and FIGS. 6(a) to 6(c) respectively illustrate a main circuit pressure, a control pressure of an electromagnetic proportional pressure reducing valve and a discharge flow rate of a hydraulic pump, with respect to a time axis (t). The hydraulic circuit illustrated in FIG. 5 includes a hydraulic pump 81, flow rate control means 83 for controlling a discharge flow rate of the hydraulic pump 81, an electromagnetic proportional pressure reducing valve 85 supplying a control pressure to the flow rate control means 83 in order to allow a discharge pressure of the hydraulic pump 81 to be a setting pressure, a relief valve 87 for reducing the main circuit pressure, and an actuator 93 connected to a main circuit 91 via a closed center type directional control valve 89.
In the hydraulic circuit described above, when using the fixed relief valve having a fixed valve opening pressure, as illustrated in FIGS. 6(a) to 6(c), if the closed center type directional control valve 89 is operated to close a valve in order to suddenly stop the actuator 93, the setting pressure of the hydraulic pump 81 is reduced. Accordingly, the control pressure of the electromagnetic proportional pressure reducing valve 85 is rapidly reduced. As a result, the flow rate control means 83 is operated to decrease the discharge flow rate of the hydraulic pump 81. At this time, the closed center type directional control valve 89 is quickly closed, but in the discharge flow rate of the hydraulic pump 81, there occurs a delay from when the electromagnetic proportional pressure reducing valve 85 responds thereto until the discharge flow rate is decreased in order for the flow rate control means 83 to be operated. Then, during the period until the discharge flow rate of the hydraulic pump 81 is decreased, although the closed center type directional control valve 89 is closed, control oil is discharged in the main circuit 91 by the hydraulic pump 81. Accordingly, the surge pressure which is momentarily high in the main circuit 91 is generated. Since the fixed relief valve 87 is installed as a safety valve of the hydraulic circuit, in which the valve opening pressure is set to have a high value, it cannot cope with the surge pressure.
In contrast, a hydraulic circuit has been proposed which uses a pilot type variable relief valve controlling the setting pressure of the hydraulic pump by using the control pressure of the electromagnetic proportional pressure reducing valve and receiving the control pressure as a pilot pressure. Specifically, as illustrated in FIG. 7, in a variable capacity type pump including a servo piston mechanism for changing a tilt angle, a controller of the variable capacity type pump is disclosed which includes a differential-pressure spool valve 102 interposed in an oil passage from a large servo chamber 101 of the servo piston mechanism disposed in the oil passage leading to a discharge oil passage, receiving the discharge pressure and the control pressure, and controlling a hydraulic pressure of the large servo chamber 101 so as to increase the tilt angle according to decrease in a differential pressure between the discharge pressure and the control pressure; an electromagnet proportional pressure reducing control valve 103 receiving the discharge pressure, reducing the discharge pressure and generating the control pressure corresponding to a solenoid drive current; pressure detection means 104 converting the discharge pressure into an electrical signal; flow rate detection means 105 converting the discharge flow rate into the electrical signal; control means (controller) 106 receiving an output of both of the above-described detection means, receiving a pressure setting signal and a flow rate setting signal, controlling and outputting the solenoid drive current based on deviation of one whose tilt angle is smaller, out of the deviation between the pressure setting signal and the detected pressure and the deviation between the flow rate setting signal and the detected flow rate; and a pilot type relief valve 107 connected to the discharge oil passage, receiving the control pressure as a pilot pressure, and capable of changing the setting pressure (valve opening pressure)(refer to JP-A-4-143471).